1. Field of the Invention
The present invention is in the field of luminescence detecting devices suitable for use in a color cathode ray tube of the beam-index type for detecting fluorescence from an index phosphor forming part of the phosphor screen of such devices.
2. Description of the Prior Art
A reflex color cathode ray tube arrangement of the beam-index type previously proposed may include a flat glass envelope which can be adapted generally to form color television receivers of relatively small size. The reflex color cathode ray tube of the beam-index type has a front panel portion which permits colored light to pass therethrough and a screen panel portion which faces the front panel portion and is provided on its inner surface with a phosphor screen including a plurality of color phosphor stripes of the three primary colors and a plurality of index phosphor stripes. Color images displayed on the phosphor screen are observed through the front panel portion and the fluorescence from each of the index phosphor stripes, i.e., the index fluorescence, is received by a photodetecting portion provided at the outside of the screen panel portion in relation to the phosphor screen.
Prior art structures of the type described are shown in FIGS. 1 to 4, inclusive. FIGS. 1 and 2 show an example of a reflex color cathode ray tube arrangement of the beam index type which is provided with a flat glass envelope 19 comprising a front panel portion 11, a screen panel portion 13 facing the front panel portion 11, and a neck portion 17 which is connected through a funnel portion 15 to both the front panel portion 11 and the screen panel portion 13. The front panel portion 11 is of rectangular shape and allows colored light to pass therethrough. The screen panel portion 13 is also of rectangular shape and is provided on its inner surface with a phosphor screen 21 extending in a rectangular pattern along the screen panel portion 13. The phosphor screen 21 includes a plurality of color stripes of the three primary colors and a plurality of index phosphor stripes. In the neck portion 17, an electron gun assembly 23 is located to produce an electron beam modulated in accordance with the color video signals supplied thereto. When the phosphor screen 21 is scanned by the electron beam from the electron gun assembly 23 on the side of the front panel portion 11, color images are displayed on the phosphor screen 21 and are observed through the front panel portion 11.
As shown in FIG. 3, the phosphor screen 21 is formed with a plurality of index phosphor stripes 29 which are fixed on the inner surface of the screen panel portion 13 and extend parallel to one another at regularly spaced intervals. An inorganic layer 31 is fixed on the inner surface of the screen panel portion 13 and extends between each adjoining pair of the index phosphor stripes 29. A thin metallic layer 33 which covers the index phosphor stripes 29 and the inorganic layer 31, and a plurality of color phosphor stripes of the three primary colors containing green phosphor stripes 35G, red phosphor stripes 35R and blue phosphor stripes 35B are fixed on the thin metallic layer 33 in a predetermined arrangement. The green phosphor stripes 35G, red phosphor stripes 35R and blue phosphor stripes 35B extend parallel to each other adjacent each of the index phosphor stripes 29 at regularly spaced intervals, and each adjoining pair of the green phosphor stripes 35G, red phosphor stripes 35R and blue phosphor stripes 35B is located between adjoining two of the index phosphor stripes 29. Each of the index phosphor stripes 29 is made of a phosphor which emits fluorescence with a peak at the range of ultraviolet light in spectral characteristics and having a short persistence characteristic, such, for example, as Y.sub.2 SiO.sub.5 :Ce.
At the outside of the screen panel portion 13 there is a luminescence sensitive plate member 25 disposed to face and extend along the outer surface of the screen panel portion 13, and a fluorescence detector 27 containing a photosensitive device such as a photodiode is attached to one end of the luminescence receiving plate member 25. Fluroescence which is emitted by each of the index phosphor stripes 29, i.e., index fluorescence, passes through the screen panel portion 13 and is operative to produce a secondary index fluorescence which is suitable for detection by the photosensitive device contained in the fluorescence detector 27. The luminescence receiving plate member 25 is formed, for example, of an acrylic resin in which a specific phosphor designed to absorb the index fluorescence from the index phosphor stripes 29 and emit a secondary index fluorescence is dispersed.
As shown in FIG. 4, when index fluorescence Li from each of the index phosphor stripes 29 enters into the luminescence receiving plate member 25, the phosphor which is dispersed in the luminescence receiving plate member 25 is excited by the index fluorescence Li and emits fluorescence. Although a portion indicated at l.sub.1 exits the luminescence receiving plate member 25 and another portion l.sub.2 of the fluorescence obtained from the phosphor dispersed in the luminescence receiving plate member 25 proceeds in the luminescence receiving plate member 25 in the opposite direction to the fluorescence detector 27, the remainder, l.sub.3, of the fluorescence obtained from the phosphor in the luminescence receiving plate member 25 proceeds directly or with total reflections at the opposite surfaces of the luminescence receiving plate member 25 toward the fluorescence detector 27 to be handled the same as a secondary index fluorescence.
The fluorescence detector 27 produces an output signal from the secondary index fluorescence appearing in the luminescence receiving plate member 25 and this output signal or a signal reformed on the basis of the detection output signal is used as an index signal for causing the electron beam emitted from the electron gun assembly 23 to be modulated with a color video signal supplied to the electron gun assembly 23 in response to the momentary scanning positions of the electron beam on the phosphor screen 21.
The conversion of the index fluorescence from each of the index phosphor stripes 21 into secondary index fluorescence produced in the luminescence receiving plate member 25 is required because the wavelength of the index fluorescence from each of the index phosphor stripes 29 themselves is not appropriate to match the wavelength sensitive characteristics of the photosensitive device contained in the fluorescence detector 27. For example, the index fluorescence emitted by each of the index phosphor stripes 29 when made of the posphor Y.sub.2 SiO.sub.5 :Ce has a peak level at a specific wavelength of about 400 nm and extends broadly to wavelengths on both sides of the specific peak. Since the time period of the index fluorescence obtained successively from the index phosphor stripes 29 when the phosphor screen 21 is scanned by the electron beam from the electron gun assembly 23 is very short and therefore the frequency of the detection output signal obtained from the fluorescence detector can be as high as 5 MHz, the index fluorescence does not match the characteristics of a silicon PIN diode used as the photosensitive device since ordinarily the PIN diode has a wavelength-sensitive characteristic in which its peak occurs for light having a wavelength of about 900 nm. Accordingly, the luminescence receiving plate member 25 is operative to receive the index fluorescence from each of the index phosphor stripes 29 with a peak specific wavelength of about 400 nm and must produce a secondary index fluorescence having a peak with a specific wavelength close to about 900 nm.
In the previously proposed luminescence receiving plate member 25, only a portion of the band of the index fluorescence output from the index phosphor strips 29 was effective to produce the secondary index fluorescence, and therefore the efficiency of conversion of index fluorescence from the index phosphor stripes 29 into secondary index fluorescence was undesirably low. Further, it is difficult as a practical matter to convert the index fluorescence from the stripes 29 directly into a secondary index fluorescence which has a peak level close to 900 nm so the secondary index fluorescence is not sufficiently close to 900 nm. This results in a low conversion efficiency and a possibility that the detection output signal from the secondary index fluorescence is insufficient to properly actuate the fluorescence detector 27 and, as a result, the color images would not be displayed properly on the phosphor screen.